Stabilized pentaerythritol ester compositions



S tates ate Fine

STABILIZED PENTAERYTHRITOL ESTER COMPOSITIONS Louis Mehr, Brentwood, Mo, and George H. Dick, East Alton, 111., assignors to Shell Development Company, New York, N. Y., a corporation of Delaware No Drawing. Application June 11, 1956, Serial No. 590,356

6 Claims. (Cl. 252-475) The invention is directed to improved pentaerythritol ester compositions. More particularly, it is directed to the stabilization of said esters for use at elevated temperatures, and the like.

Active development is proceeding in the field of synthetic lubricants and plasticizers for the production of compositions having improved stability required by increasing operating temperatures, both of lubricants and of plasticized compositions such as rubbers and the like. The stability, particularly where metals are in contact with the ester, is a complex problem concerned not only with attack of the metal by decomposition products of the ester, but also concerned as much with changes in the physical propertiesof the ester as it is decomposed at elevated temperatures due either to thermal degradation or to oxidation. Numerous stabilizing substances have been investigated to overcome the various phenomena encountered during use of aliphatic esters at elevated temperatures. Many of these are quite effective at relatively mild elevated temperatures but as the temperatures are increased beyond about 350 F., and particularly in the range of 400 5., stabilizers found to be efiective at lower temperature rapidly lose their stabilizing efiect and in numerous instances actually become pro-oxidants.

It is an object of the present invention to improve the stability of pentaerythritol esters of a certain class for use at elevated temperatures. It is a further object of this invention to provide improved high temperature lubricating compositions. It is another object of the invention to provide stable plasticizing compositions. It is a particular object of this invention to provide stable gas turbine lubricants showing a minimum tendency to increase in viscosity and acid number during use at elevated temperatures. Other objects will become apparent during the following description of the invention.

Now, in accordance with the present invention, it has been found that esters of pentaerythritol wherein each ester radical contains from to 7 carbon atoms can be stabilized with respect to decomposition at temperatures in the order of 350425 F. by the synergistic combination of certain diarylamines with certain thiodiarylamines, wherein each aryl group is a phenyl, naphthyl, alkylated phenyl or alkylated naphthyl radical. The combinations which have been found to exhibit the most striking synergism in this respect are as follows: (1) diphenylamine with phenothiazine, (2) phenyl-alphanaphthylamine with phenylthio-alpha-naphthylamine, (3) phenyl-betanaphthylamine with phenylthio-beta-naphthylamine, (4) betabeta-dinaphthylamine with phenothiazine, and (5) beta, beta-dinaphthylamine with beta, beta-thiodinaphthylamine. Other suitable diarylamines include methylphenylalpha-naphthylamine and phenyl-beta-(butylnaphthyl) amine. Further thiodiarylamines include phenylthioalpha-(methylnaphthyl)-amine and thio(diethylphenyl)- amine.

The synergism exhibited by these pairs of additives has een found to be quite specific in that addition of the above pairs of antioxidants to pentaerythritol aliphatic esters other than those called for above does not result in any effective stabilization of the esters.

The pentaerythritol esters which may be stabilized by one or more of the above-described pairs of additives include especially pentaerythritol tetracaproate, pentaerythritol tetravalerate, pentaerythritol tetraheptoate, pentaerythritol dicaproate divalerate, pentaerythritol tricaproate heptoate, and mixtures thereof. In order to obtain the maximum result with respect to stabilization, it is preferred that the pentaerythritol esters given above be present in a major amount and preferably in an amount greater than about by weight of the total composition. When said pentaerythritol esters are present in a major amount, other esters or diluents may be present in addition thereto, including other pentaerythritol esters, aliphatic esters of dicarboxylic acids and aliphatic esters of aromatic acids. Suitable species of such diluting materials include bis(2-ethylhexyl)sebacate, dibutylphthalate, silicones such as the methylphenyl silicones or dimethyl silicones, silicates and particularly the aliphatic silicates such as tetrabutyl silicate, phosphorus esters including phosphates, phosphonates and phosphinates such as tricresyl phosphate, trioctyl phosphate, triethyl phosphate, dihexylhexane phosphonate and the like. It is preferred that the proportion of diluting fluids be present in an amount less than about 25% by weight of the total composition.

The proportion of stabilizer to be added to the recited class of pentaerythritol esters should be between about 0.1% and about 2.5% of each of the stabilizing materials. It is preferred, however, that the proportion of stabilizer, that is, of either the diarylamine or of the thiodiarylamine, be between about 0.25% and about 1.5% each. Expressed in terms of moles of stabilizing material, it is preferred that the stabilizers be present in an amount between about 1.5 and about 5 moles X10 per grams of the pentaerythritol ester.

The synergism exhibited by the above recited pairs of stabilizing materials in the recited class of pentaerythritol esters is not with respect to improvement in the corrosion characteristics of said esters. Furthermore, the effect noted is not normally evident below temperatures of about 350 F. Consequently, the described compositions are especially surprising in their highly stable nature at temperatures above about 350 R, such as those encountered in the operation of gas turbines and, particularly, of aircraft gas turbine engines. While the corrosion charac teristics of the lubricants are only mildly improved by the presence of the combination of stabilizing substances, there is a surprising improvement both in the percentage of viscosity increase during use of the esters at high temperatures and in the acid number resulting from such use. These phenomena are especially striking in view of the fact that other pentaerythritol esters do not respond to the combinations of additives effective in the class of esters being claimed nor do other closely related diarylamines or thio derivatives thereof in combination synergistically improve the stability of the acid number or viscosity of the recited class of pentaerythritol esters during use at elevated temperatures. These points will be demonstrated by means of comparative data contained in the examples which follow.

While the compositions described are especially useful as lubricating substances at elevated temperatures, they may, as intimated above, be utilized as plasticizers for rubbers and plastic materials and especially for vinyl plastics.

In the examples which follow the conditions under which the compositions were tested are those described in the corrosion-oxidation portion of Military Specification MIL-L-7808C, with the exception that temperatures of 3 400 F.'were utilized instead of the temperatures of about 350 F. called for by the specification. In this test, as is well known to experts in this art, the material is heated under oxidizing conditions in the presence of metallic test strips. The weight gain or loss of the strips is determined after the test and the neutralization number of the fluid and change in viscosity thereof after the test are determined. Table I below illustrates the synergistic combinations by means of comparative test results with similar compositions in which the single additives are present to the exclusion of the second member of the recited pairs, the total amount of additive present in synergistic combination being the same on a molar basis as that present when the individual additives are employed alone.

The described combination of inhibitors is efiective for the stabilization of mixtures of esters as stated hereinbefore. For example, the combination of 50% pentaerythritol tetrocaproate with 48% bis(2-ethylhexyl)sebacate stabilized with about 1% each of phenyl-beta-naphthylamine and phenylthiobeta-naphthylamine showed substantially no corrosion toward the metal strips utilized in the test described with reference to Table I and at the end of the test it had a neutralization number of only 2.0. The stabilization of a mixture of the same esters in a proportion of 4 parts of the tetracaproate to 1 part by weight of the sebacate with the same additives resulted in both a low neutralization number (2.6) and a low percentage increase in viscosity (7.1%) at the end of the test.

We claim as our invention:

TABLE I Corroszon-Oxzaatzon Test, 72 h!., 400 F .M ed: fled M IL-L7808C Weight Loss or Gain in MgJCm. N ent Visc. Additive Percent w. MoleX 10- N 0. Percent Per 100 g. 2.0 5/+15 Mg:l;0.20 A1:1:0.20 011:1:040 Fe:|=0.20 Agi0.20

Diphenylamine+Phenothiazine. 1. +0. 5. 9+2. 5 +0. 03 +0. 07 0. 02 +0. 06 +0. 02 1. 2 22. 9 Diphenylamine 1.05 6. 2 0 0 O. 68 +0.02 O. 04 2. 6 53.0 Phenothiazine 1. 24 6. 2 +0.09 +0. 0 +0. 10 +0. 04 2. 2 20. 1 Phenyl-a-Naphthylamine Phenylthio-aNaphthylamine O. 81+0. 63 3. 7+2. 5 +0.06 +0.05 O. +0.05 +0. 03 1. 7 16. 8 Phenyl-a-Naphthylarnine 1. 36 6. 2 0 0 O. 70 O. 08 2. 2 41. 7 Phenylthio-a-Naphthylamine 1. 6. 2 +0.01 +0.03 1. 08 +0. 05 +0.02 5.0 25.6 Ph enyl-B-Naphthylamine+ Phenylthio-fi-Naphthylamine- 0. 81+0. 63 3. 7+2. 5 0 0 0. 43 --0. 03 O. 03 1. 7 8. 6 Phenyl-B-Naphthylamine 1. 36 6. 2 0 0 -0. 08 +0. 02 O 1. 1 18.0 PheuylthioB-Naphthylamiue. 1. 55 6. 2 +0. 10 +0. 17 1. 05 +0. 08 +0. 10 3. 7 24. 5 fl,B-Dinaphthylamine+Phenothiazine 1+0. 5 3. 7+2. 5 +0.03 +0. 04 +0. 01 0 1. 6 17. 6 Phenothiazine 1. 24 6. 2 +0. 09 +0. 15 0 +0. 10 +0. 04 2. 2 20. 1 ,8,B-Dinaphthylamine 1. 67 6. 2 +0. 05 +0.05 0. 04 +0. 01 +0.02 4. 0 29. 8 8,B-Dinaphthy1amine+fl,fi-

Thiodinaphthylamiue 1+0. 75 3. 7+2. 5 +0. 01 +0. 04 2. 09 +0.03 0. 05 1. 6 14. 5 @B-Thiodinaphthylamine 1. 86 6. 2 +0. 01 +0.07 4. 12 0 +0. 07 3. 7 29.0 5,6-Dinaphthy1arnine l. 67 6. 2 +0.05 +0.05 O. 04 +0.01 +0.02 4. 0 29. 8

In order to illustrate the unexpected character of the above-demonstrated synergism, the same pentaerythritol tetracaproate, which was employed in the tests described in Table I, was inhibited by other pairs of diarylamines and thiodiarylamines. In most instances, the individual additives employed are those utilized in other combinations as described in Table 1.

Having found that the pentaerythritol esters having from 5 to 7 carbon atoms per ester radical could be stabilized with respect to viscosity increase and acid number after being subjected to high temperatures, the stabilization of other pentaerylthritol esters was attempted using one of the combinations of inhibitors found to be successful in the tests described in Table I. The combination employed was 1% diphenylamine and 0.5% phenothiazine. Pentaerythritol dilaurate dibutyrate was modified with this combination, as was pentaerythritol tetra(2-ethylhexoate). These compositions were subjected to the oxidation-corrosion test utilized in gathering the data described in Table I. It was found that the modified pentaerythritol dilaurate dibutyrate composition formed a heavy deposit on the magnesium test metal strip and developed an unsatisfactorily high acid number. Moreover, the composition deposited a heavy sludge under the test conditions making it impossible to obtain a satisfactory viscosity on the sample after test.

The composition comprising pentaerythritol tetra(2- ethylhexoate) did not corrode the test metal strips appreciably, but formed an acid number of 5.8 and showed a 71% increase in viscosity after the test. Consequently, it will be seen that the combination of inhibitors is highly specific in its action and is not effective for the two pentaerythritol esters just described. It will be noted that the ester radicals present in each of these two pentaerythritol derivatives were either longer or shorter than those which could be effectively stabilized by the recited pairs of stabilizing material described hereinbefore,

1. A composition consisting essentially of a major amount of pentaerythritol tetracaproate and minor amounts between about 0.1% and about 2.5% by weight, suflicient to substantially minimize viscosity increase and acidity increase at elevated temperatures of both phenothiazine and diphenylamine.

2. A composition consisting esentially of a major amount of pentaerythritol tetracaproate and minor amounts between about 0.1% and about 2.5% by weight, sufiicient to substantially minimize the increase in viscosity and acidity thereof due to thermal and oxidative degradation at high temperatures, of both phenothiazine and beta, beta'-dinaphthylamine.

3. A composition consisting essentially of a major amount of pentaerythritol tetracaproate and minor amounts between about 0.1% and about 2.5% by weight, sufiicient to substantially minimize the increase in viscosity and acidity thereof due to thermal and oxidative degradation at high temperatures, of phenyl-alpha-naphthylamine and phenylthio-alpha-naphthylamine.

4. A composition consisting essentially of a major amount of pentaerythritol tetracaproate and minor amounts between about 0.1% and about 2.5% by weight, sufficient to substantially minimize the increase in viscosity and acidity thereof due to thermal and oxidative degradation at high temperatures, of phenyl-beta-naphthylamine with phenylthio-beta-naphthylamine.

5. A composition consisting essentially of a major amount of pentaerythritol tetracaproate and minor amounts between about 0.1% and about 2.5 by Weight, sufiicient to substantially minimize the increase in viscosity and acidity thereof due to thermal and oxidative degradation at high temperatures, of beta,beta'-dinaphthylamine with beta,beta'-thiodinaphthylamine.

References Cited in the file of this patent UNITED STATES PATENTS 2,609,343 Saunders et al. Sept. 2, 1952 2,691,632 Harle Oct. 12, 1954 2,721,177 Harle Oct. 18, 1955 2,722,518 Watson Nov. 1, 1955 OTHER REFERENCES Ind. and Eng. Chem., vol. 42, No. 12, December 1950, pages 2479-2489. 

6. A COMPOSITION CONSISTING ESSENTIALLY OF A MAJOR AMOUNT OF A PENTAERYTHRITOL TETRAESTER OF A SATURATED FATTY ACID HAVING FROM 5 TO 7 CARBON ATOMS PER MOLECULE AND MINOR AMOUNTS BETWEEN ABOUT 0.1% AND ABOUT 2.5% BY WEIGHT, SUFFICIENT TO SUBSTANTIALLY MINIMIZE THE INCREASE IN VISCOSITY AND ACIDITY THEREOF DUE TO THERMAL AND OXIDATIVE DEGRADATION AT HIGH TEMPERATURES, OF BOTH A DIARYLAMINE AND A THIODIARYLAMINE, WHEREIN EACH ARYL RADICAL IS SELECTED FROM THE GROUP CONSISTING OF PHENYL, NAPHTHYL, ALKYLATED PHENYL AND ALKYLATED NAPHTHYL RADICALS ANY ALKYL SUBSTITUENTS FORMING A PART OF SAID RADICALS HAVING FROM 1 TO 4 CARBON ATOMS EACH. 